This invention relates to a field mounted measurement transmitter measuring a process variable representative of a process, and more particularly, to such transmitters which have a microprocessor.
Measurement transmitters sensing two process variables, such as differential pressure on either side of an orifice in a pipe through which a fluid flow, and a relative pressure in the pipe, are known. The transmitters typically are mounted in the field of a process control industry installation where power consumption is a concern. Other measurement transmitters sense process grade temperature of the fluid. Each of the transmitters requires a costly and potentially unsafe intrusion into the pipe, and each of the transmitters consumes a maximum of 20 mA of current at 12 V. In fact, each intrusion into the pipe costs between two and seven thousand dollars, depending on the types of pipe and the fluid flowing within the pipe. There is a desire to provide measurement transmitters with additional process measurements, while reducing the number of pipe intrusions and decreasing the amount of power consumed.
Gas flow computers sometimes include pressure sensing means common to a measurement transmitter. Existing gas flow computers are mounted in process control industry plants for precise process control, in custody transfer applications to monitor the quantity of hydrocarbons transferred and sometimes at well heads to monitor the natural gas or hydrocarbon output of the well. Such flow computers provide an output representative of a flow as a function of three process variables and a constant containing a supercompressibility factor. The three process variables are the differential pressure across an orifice in the pipe containing the flow, the line pressure of the fluid in the pipe and the process grade temperature of the fluid. Many flow computers receive the three required process variables from separate transmitters, and therefore include only computational capabilities. One existing flow computer has two housings: a first housing which includes differential and line pressure sensors and a second transmitter-like housing which receives an RTD input representative of the fluid temperature. The temperature measurement is signal conditioned in the second housing and transmitted to the first housing where the gas flow is computed.
The supercompressibility factor required in calculating the mass flow is the subject of several standards mandating the manner and accuracy with which the calculation is to be made. The American Gas Association (AGA) promulgated a standard in 1963, detailed in "Manual for the Determination of Supercompressibility Factors for Natural Gas", PAR Research Project NX-19. In 1985, the AGA introduced another guideline for calculating the constants, AGA8 1985, and in 1992 promulgated AGA8 1992 as a two part guideline for the same purpose. Direct computation of mass flow according to these guidelines, as compared to an approximation method, requires many instruction cycles resulting in slow update times, and a significant amount of power consumption. In many cases, the rate at which gas flow is calculated undesirably slows down process loops. Cumbersome battery backup or solar powered means are required to power these gas flow computers. One of the more advanced gas flow computers consumes more than 3.5 Watts of power.
There is thus a need for an accurate field mounted multivariable measurement transmitter connected with reduced wiring complexity, operable in critical environments, with additional process grade sensing capability and fast flow calculations, but which consumes a reduced amount of power.